In the operation of a fluid system serving a plurality of work elements, the work elements often demand large volumes of fluid from their associated hydraulic fluid pumps. Situations arise where the work elements demand fluid at a rate greater than the capacity of the hydraulic pump, thus flow limitation or "saturation" occurs. Full saturation occurs at the point at which the pump pressure is substantially the same as the pressure at the work element.
Depending upon the specific machine application, the flow demands of the work element may exceed the flow capacity of the pump or pumps if the hydraulic system remains in a fixed element priority. In this state, control of the work elements is severely limited. Attempts by the operator to adjust the inputs correctly to avoid or overcome this state often lead to poor production. For example, as an agricultural tractor using a seed spreading implement approaches the end of a row and prepares to turn, the seed spreading implement is raised as the implement continues to spread the seed. If the demand on the electro-hydraulic pump is greater than what it can produce, other valves will be "starved" as pump flow is rerouted to the valve controlling the lifting of the seed spreading implement. An area of unseeded land is produced if one of the valves being starved is the valve controlling the spreading of the seed. The process of starving some more heavily loaded valves to supply pump flow to a valve under a lower load demand is a result of a saturation condition. In addition, automatic functions, such as an auto dig cycle for an excavator, cannot be implemented on such a machine. When saturation occurs during an automatic function cycle, the machine stalls or incorrectly performs the function.
Known attempts to address the problem of flow saturation include a priority system and a pressure compensation system. The former solution gives priority to one valve over all other valves in a given system. One of the valves receives pump flow while the other valves are starved. For example, a plurality of control valves can pass fluid from a pump to a plurality of respective work elements. A preprogrammed controller responsively determines the priority in which fluid is distributed from the pump to the respective control valves and then the controller delivers control signals to the respective control valves in response to the determination to control the amount of flow through each valve, for example, by selectively positioning the stem of the respective control valves. In the latter solution, the pressure compensation system, there is proportional scaling back of flow to all control valves in the system when the load demand exceeds what the hydraulic pump can supply.
Other known systems of dealing with saturation include simply increasing pump speed in an attempt to provide more output flow to the valves. None of the known methods for addressing the problem of saturation has confronted the saturation problem before the situation has fully occurred, that is, previous methods have not attempted to predict a saturation condition and limit fluid flow to prevent valve starvation. Rather, the problem of saturation has conventionally been addressed only after the condition has completely developed and valves are being starved, either fully or in part.
Accordingly, the present invention is directed to overcoming one or more of the problems as set forth above.